This invention relates to a method in the treatment of wastewater from the manufacture of mechanical or chemi-mechanical cellulose pulp at closed manufacturing processes. The treatment is carried out according to the invention in two steps, viz. a hydrolysis and acid fermentation step and a methane fermentation step.
Both of said steps are carried out as completely agitated contact reactor processes, and both steps are followed by a subsequent sludge separation step, by which the main part of separated sludge is recycled to the respective contact reactor. According to an important embodiment of the method according to the invention, long fibres are to a substantial part removed from the wastewater.